The invention relates to the field of bolt tightening by cold preload, and in particular relates to a method of checking the preload in the shank of the bolt, at the end of tightening. The invention also relates to a device for implementing a method such as this.
Bolts comprising a shank, of the screw or stud type, and one or more nuts, are used to assemble numerous components of a structure. It is known practice, in order to achieve quality tightening, for a tightening shank to be stretched using a hydraulic ram and for a tightening nut then to be brought into contact with the components that are to be tightened, without applying significant torque, and then for the extensile load on the shank to be released. The released shank tends to return to its at-rest length but, being prevented from doing so by the nut, applies a clamping force to the components that are to be tightened. A preloading method such as this avoids nuts having to be tightened by applying tightening torques to them.
Furthermore, in order to tighten the circular flange, various bolts distributed around the flange that is to be tightened are tightened alternately. Tightening in this way requires a number of tightening steps for each of the bolts. What actually happens is that the tightening of one bolt relieves the stress in the neighboring bolts. The applicant has discovered that the number of steps required to tighten each bolt increases rapidly with the number of bolts distributed around the circular flange that has to be tightened.
By way of an example of the tightening of a number of bolts, mention may be made of the case of rolling bearings measuring several hundredths of millimeters or even several meters in diameter, which are needed for attaching wind generator blades to a central hub of the rotary of the wind generator or for attaching the directional electric generator equipped with the wind generator rotor to the top of the stationary mast. One of the rings of these enormous rolling bearings is generally factory-fitted onto part of the equipment, the other ring being connected to a circular tightening flange of the same size. This tightening flange may be assembled with the rotating part of the equipment on the site on which the wind generator is ultimately assembled. Attaching just one of these flanges may require in the order of one hundred bolts.
Unwanted loosening of just one of the numerous bolts has a knock-on effect on the other bolts and carries the risk of causing warping and loadings of the flange, causing other bolts to exceed their elastic limit. This may, through a domino effect, lead to deterioration of the entire fastening. Often, given uncertainties regarding tightening, particularly when it is tightening to torque that is being employed, the bolts and nuts are over engineered. This does not improve the accuracy of the tightening but it does increase the dimensions and mass of the assembly, which is particularly penalizing in the case of equipment that has to be assembled at a great height.
Document FR 2 841 304 (SKF) describes a device comprising an annular plate equipped with a plurality of circumferentially distributed hydraulic devices. This device allows global tightening of a plurality of threaded shanks distributed around a circular flange, for example. Tension is simultaneously applied to all the threaded shanks, tightening nuts are brought into contact on each of the shanks with the component that is to be tightened then the tensions are simultaneously released. Simultaneous tensioning is particularly beneficial for tightening circular flanges in which the number of bolts to be tightened is very high. Simultaneous tensioning of the bolts avoids the alternate tightening of each of the bolts in multiple stages. The disadvantage with such a method is that the tension that remains in each bolt following the simultaneous release of the shanks is dependent on the positioning of the tightening nut. If one of the tightening nuts has not been positioned correctly in contact with the flange that is to be tightened while the shank was stretched, the tension in the tightened part of the bolt may be very low, even though the extensile tension was checked and the nut appeared to be tight. It is therefore necessary to have a means of checking the tightening obtained.
Document FR 2 871 231 (SKF) describes a method of checking the residual tension in a working part of a shank of the screw or stud type delimited by a nut. This check is performed after the shank has been tensioned, by axial stretching, the nut has been fitted and the shank has been released. This method of checking consists in measuring the elongation of the shank as a function of the extensile force. The disadvantage with this method of checking is that it is individual for each given bolt and that the shank has to be re-tensioned specifically for the purpose of the checking operation.
That document also mentions another method of checking that uses gauged bolts to determine the residual elongation of the shank of the bolt after release. A reference gauging rod is positioned in an axial passage machined in the center of the bolt that is to be checked. In this method, the difference in position between the top of the gauging rod and the top of the bolt is compared before and after tightening using a feeler. This method is expensive, is restricted to individual checking of shanks, and usually entails removing the tensioning device in order to perform the checking step. Another method is to position an ultrasound probe on the bolt shank before and after the preloading. That method requires specialist operators and has to be applied to each bolt. That is expensive. Another method is to fit the bolts with a sensor washer capable of measuring the load in the bolt. This checking means remains permanently fitted to the bolt that is to be checked.
Document DE 28 46 668 describes an elongation measurement instrument for a stud tightening device. The stud has an axial bore through which a gauging rod passes. The gauging rod butts via its end against a nut fixed in the lower part of the stud. An elongation measurement instrument is positioned at the top of the gauging rod and bears against the top face of the stud. A method such as this for checking the preload has the disadvantage of requiring special purpose studs that have to be fitted with a gauging rod passing through them. That reduces the working cross section of the stud or studs. It is desirable to be able to check the preload established in one or more standard studs.